The invention relates to an apparatus and method for compressing a sequence of digital images. It finds particular application in conjunction with compressing the sequence of digital images, such as from a surveillance camera or other video stream in which all or a portion of the images remain relatively static and will be described with particular reference thereto. However, it is to be appreciated that the invention is also amenable to other applications.
As is well known in the prior art, a video image, when digitized, requires a large amount of storage. A plurality of video images (as used hereinafter: “a video scene”), such as a movie, requires hundreds of megabytes or even gigabytes of storage, if not compressed.
Various methods to compress a video scene are known in the prior art (e.g., MPEG1, MPEG2, etc). One prior art method is to derive a parameter by principal component analysis for all the images of the video scene. An image value based upon the parameter chosen by principal component analysis is determined for each of the video images. The resultant storage requirement, for one parameter, is one image full of component values for that parameter (N values for an image with N pixels) and one image value associated with each video image. If more than one parameter is derived for the video images, the total amount of storage required is multiplied correspondingly.
Surveillance cameras often run at a slow frame rate. At regular video rates, e.g., thirty images per second, a two hour video has over 200,000 video images and uses about 50 gigabyte of storage. The principal component analysis method, for one parameter, derives N parameter component values for each image.
Another method of compression of the prior art is to choose initial images as a reference frame. Subsequent images are compared to the reference frame, and only the differences or changes are stored. Thus, complete data is stored for each reference frame, but the changes are stored for subsequent images.
In such prior art method, the reference frames are chosen based upon every nth frame. Thus, the choice of the reference frame is not optimized.
U.S. Pat. No. 6,009,204 to Ahmad discloses a method of compressing a plurality of video images for storing, displaying and searching the plurality of video images. In the method, a video image, characterized by a plurality of pixels, is compressed by deriving at least one parameter based upon principal component analysis. An image value for each video image is then determined for the one parameter. The image value associated with each video image is compared to an image value associated with another video image to determine a difference. When the difference between image values of compared video images is below a threshold, one of them is discarded. The remaining video images are then stored. Using this method of compression, it becomes possible to search a video database to find a match between an inquiring video scene with a video database made up of a plurality of video scenes with each video scene having a plurality of video images.
Security cameras typically record incidents, e.g., a thief entering a shop, while non-relevant video images of the empty shop are discarded without being recorded. This reduces the storage requirement for image data at the loss of continuous recording.
As the use of video images to capture information becomes more prevalent and the continuous need for higher image quality leads to higher resolution and increased image parameters, it is increasingly advantageous to efficiently store and display a large plurality of video images, or the video scene with minimal storage space.